Search

Signals-of-Opportunity (SoOp) reflectometry is a concept for reutilizing existing microwave satellite transmissions, in bands allocated for space-to-Earth communications and navigation, for the purpose of Earth remote sensing. SoOp uses a bistatic radar geometry employing forward scatter, which views Earth differently from traditional monostatic (backscatter) radars and radiometers. Additionally, SoOp enables use of microwave frequencies outside the limited number of narrow and protected bands allocated for remote sensing. A powerful example of the science returns achievable using SoOp is currently being demonstrated on orbit by the NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission, which measures ocean surface wind speed using reflected Global Positioning System (GPS) signals. Over the past decade we have seen a growing number of communication and navigation signals coming into existence spanning the range of frequencies from Very High Frequency (VHF) to K Bands, which could be used to measure different variables using wavelengths optimally suited to specific interactions with the Earth system, including: Soil Moisture; Sea Surface Height; Ocean Vector Winds; Snow Water Equivalent; Root Zone Soil Moisture; and Vegetation Water Content. The use of SoOp for earth remote sensing could enable these measurements from small passive receivers in Low Earth Orbit (LEO) on CubeSat constellations to improve the temporal frequency of the observations. This paper describes different architectures of CubeSats constellations that could be used to measure different science measurements as well as identifying some of the challenges of the SoOp technique.

Recently, there has been increasing interest in the use of spaceborne synthetic aperture radar (SAR) for measuring forest biomass. However, it is noted that conventional SAR using C-band or higher frequencies cannot penetrate into foliage, and therefore the biomass measurements require longer wavelengths, typically P-band (500 MHz). It is also known that the ionosphere is highly dispersive, causing group delay and broadening of pulses. The variance of the refractive index fluctuations due to turbulence is approximately proportional toƒ−4. In addition, the Faraday rotation due to the geomagnetic field in the ionosphere becomes significant. This paper presents an analysis with numerical examples of the following effects in the frequency range from 100 MHz to 2 GHz in order to show the frequency dependence and the effects of total electron content (TEC) of the ionosphere. First, the ionospheric turbulence can reduce the coherent length below the equivalent aperture size, and the azimuthal resolution becomes greater than D/2 where D is the antenna aperture size. Second, the ionospheric dispersion causes a shift of the imagery due to the group velocity. Third, the dispersion also creates broadening of the pulse. In addition, multiple scattering due to ionospheric turbulence gives rise to pulse broadening. Fourth, we consider the Faraday rotation effect and show that the ellipticity change is negligible, but the orientation angle changes significantly at P-band. Numerical examples are shown using typical ionospheric parameters, turbulence spectrum, and TEC values.

Defining an archaeological context for the temples of Angkor has been a major challenge in the interpretation of the ancient site. The authors’ collaborative research addressed this issue using data acquired with the NASA Jet Propulsion Laboratory’s Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) in 1994 and the Airborne Synthetic Aperture Radar (AIRSAR) in 1996. The first archaeological use of combined SIR-C and AIRSAR datasets in Southeast Asia, and the ensuing perceptions identified new strategies of innovative research that have fundamentally changed our understanding of the breadth and complexity of the ancient Khmer achievement. The results summarized in this article are both technical and archaeological, ranging from the analysis of polarimetric and interferometric data to the transition from prehistoric habitation to the Hindu-Buddhist cities of Angkor in the 9–13th centuries A.D.

In situ transmission electron microscopy is an established experimental technique that permits direct observation of the dynamics and mechanisms of dislocation motion and deformation behavior. In this article, we detail the development of a novel specimen goniometer that allows real-time observations of the mechanical response of materials to indentation loads. The technology of the scanning tunneling microscope is adopted to allow nanometer-scale positioning of a sharp, conductive diamond tip onto the edge of an electron-transparent sample. This allows application of loads to nanometer-scale material volumes coupled with simultaneous imaging of the material's response. The emphasis in this report is qualitative and technique oriented, with particular attention given to sample geometry and other technical requirements. Examples of the deformation of aluminum and titanium carbide as well as the fracture of silicon will be presented.

A miniature vise built into a 5 mm diameter copper capsule is described that holds small pieces of prefrozen, hydrated specimens at low temperatures within the lens of the Hitachi S900 high-resolution scanning electron microscope.

In situ transmission electron microscopy is an established experimental technique that permits direct observation of the dynamics and mechanisms of dislocation motion and deformation behavior. In this article, we detail the development of a novel specimen goniometer that allows real-time observations of the mechanical response of materials to indentation loads. The technology of the scanning tunneling microscope is adopted to allow nanometer-scale positioning of a sharp, conductive diamond tip onto the edge of an electron-transparent sample. This allows application of loads to nanometer-scale material volumes coupled with simultaneous imaging of the material’s response. The emphasis in this report is qualitative and technique oriented, with particular attention given to sample geometry and other technical requirements. Examples of the deformation of aluminum and titanium carbide as well as the fracture of silicon will be presented.